Understanding Combustion Mechanism of Magnesium for Better Safety Measures: An Experimental Study

Round 1

Reviewer 1 Report

The paper conduced experiment to study the combustion mechanism of magnesium. It is an interesting work. But the analysis of the result still needs to go further in-depth. My detailed comments are as follows.

1. It is suggested to shorten the abstract section and reformulate this section more logically. For instance, the background of the study was taken 3 paragraphs, while the literature review was only 1 paragraph. In addition, the literature review is really not sufficient.
2. The chapter division is very confusing. It is suggested to merge the section 4 and 5. The experimental procedure and the tested cases should be added.
3. The reason/specific criteria to structure the fire mechanism of magnesium should be provided.
4. It mentioned that the experiment used the ‘magnesium powder’ rather than the real ‘magnesium dross’. In the meantime, a small-scale experiment was conducted. Can the results be applied to the real scale? How is the generality of the conclusions?
5. The language needs serious improvement. Many grammar problems make it very hard to understand.

Author Response

Reviewer #1

Question # 1: It is suggested to shorten the abstract section and reformulate this section more logically. For instance, the background of the study was taken 3 paragraphs, while the literature review was only 1 paragraph. In addition, the literature review is really not sufficient.

Response #1: Thank you for your suggestion. We have reviewed through the full content of this manuscript and updated the contents following your suggestion.

First of all, we tried to shorten the abstract and reorganize more logically in lines 8-21 as follows:

“With increasing the quantity of recycling magnesium, the risks of fires and explosions have been increased. In particular, dross generated in the magnesium recycling process is not considered, as a hazardous material, even though it contains a large amount of pure magnesium and/or magnesium compound. There are few safety measures to prevent and respond to the potential fires and explosions in the magnesium recycling process and protect businesses and workers. Therefore, this study aims to identify appropriate safety measures to prevent and respond to the possible fires and explosions by looking at two actual magnesium fire cases in South Korea, relevant criteria of South Korea, the U.S., and Japan, and conducting a magnesium combustion experiment. We developed a magnesium dross combustion mechanism, including chemical reaction, smouldering, and breaking out a fire. Each stage also presents five items with safety measures in the visibility of combustion reaction, the velocity of the combustion reaction, identification methods, response measure, and possible responders. Although this study focused on dross generated from the magnesium recycling process, this study is expected to be helpful to develop safety and risk management measures to reduce the risks of various combustible metal fires and explosions.”

To fill out insufficient literature review, we have reviewed and added previous studies to support our study in lines 124-173 as follows:

Numerous studies have been carried out on assessing combustible characteristics by investigating the explosivity of magnesium dust and powder. Only a minority of studies consider fires of magnesium and dust and powder. The explosion of dust and power is sorted into the ignition and strength characteristics (Assael, 2010). Generally, the ignition characteristic is indicated by explosive range, MIT, MIE, and limiting oxygen concentration (LOC). The strength characteristic is denoted by P_max, maximum pressure rise rates (dP/dt_max), and flame propagation speed (Marc, 2010). NFPA 484 (2019) assesses explosion risk of combustible metal powder and dust with the factors of MIE, P_max, dP/dt_max, deflagration index (K_st), LOC, and MEC. Moreover, other studies related to magnesium dust explosion have addressed the explosive properties depending on different particle sizes through analysis on the ignition and strength characteristics (Nifuku et al., 2007; Li et al., 2008; Dehaan et al., 2012; Yuan et al., 2013; Mittal, 2014; Meng et al., 2020; Yashima, 2020).

Among the property factors, MIT (K) and MIE (mJ) are particularly considered as essential factors to assess explosive properties as well as fire risks. Previous studies have shown that the more the size of material particles is small, the more the MIE and MIT are decreased, which means the risk of explosion and fire is increased. For example, Nifuku et al. (2007) revealed that when magnesium particle size is 0-20 μm and 149-177 μm, the MIT and MIE is 786.15 K/4 mJ and 898.15 K/242 mJ, perspectively. Li et al. (2008) identified the MIE of magnesium particles with 6 μm is below 2mJ. Also, another experiment illustrated that MIE is more highly elevated rather than MIT, and 2 mJ of energy could be easily generated by electric sparks and fraction on the mechanical properties (Li et al., 2008). It means that the more magnesium particle is tiny, the more the risks of fire and explosion increase.

In the other study, Yuan et al. (2013) conducted an experimental study using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) to identify combustion reactions with nitrogen and oxygen gases. In an experiment using a magnesium particle of 6 μm with a temperature rise rate of 325.15 K/min, the mass (%) was slightly increased between 603.15 and 773.15 K. This increase is interpreted as the starting oxidation and nitridation and it is rapidly reacted between 773.15 - 873.15 K. During the rapid reaction, the first peak was 863.15 ℃ in DSC analysis and the rapid oxidation was continued until the melting point reached 883.15 K. And then, the speed of the oxidation is slowed until it reaches 923.15 K. It means that the magnesium combustion proceeds in the order of slow-burning until it gets autoignition temperature, rapid oxidation and nitridation, a reach for the melting point of magnesium power, and a downturn of oxidation and nitridation.

Theoretically, magnesium burning shows combustion with flams and smouldering, and the smouldering follows the magnesium melting point (Kudo et al., 2010; Neikov et al., 2019). When combustible metal, particularly magnesium, is burned, the rate of combustion is relatively high-speed, but the smouldering is continued for a longer time. Kudo (2010) evidenced that if the particle size is smaller, the flame spread rate dramatically increases. In addition, the more the size of the particle is small, the more the layers of oxidation and melting are thinner. This resulted that the combustion reaction being promptly expanded and transferring the heat of combustion is comparatively delayed if the particle size is small, and it takes a longer time to end up the combustion due to expanded smouldering. So far, most previous studies have addressed the combustible risks of magnesium as a metallic powder by looking at the phenomena of its explosive reaction; however, there have been few studies on fires of combustible metals, such as magnesium, even though the mechanism of combustible metal fires and explosions is different.

Question # 2: The chapter division is very confusing. It is suggested to merge the section 4 and 5. The experimental procedure and the tested cases should be added.

Response #2: We rechecked Section 4 and 5. Following your suggestions, we merged both section to Section 4. Also, it includes the experimental procedures in lines 367-374 as follows:

“The experiment is conducted indoors in order to control the external environmental conditions. The procedure is as follows: 1) set up the experiment equipment on the flat ground; 2) put 11 Kg of pure magnesium powder on the plasterboard in the experiment bed; 3) ignite the top and bottom of the magnesium powder dummy using a gas torch for 30 sec; 4) leave the burning magnesium powder dummy until it is suppressed by itself; and 5) collect the internal and external by-products from the area that the burning is started. To prevent possible accidents, we prepared safety measures, such as fire extinguishers and personal safety devices.”

Question #3: The reason/specific criteria to structure the fire mechanism of magnesium should be provided.

Response #3: Thank you for your comments. Since there are no appropriate criteria, we referred some literature that is reviewed in Subsection 2.1 and what we found from this study. The contents are indicated in lines 159-171 as follows:

“Theoretically, magnesium burning shows combustion with flams and smoldering, and the smoldering follows the magnesium melting point (Kudo et al., 2010; Neikov et al., 2019). When combustible metal, particularly magnesium, is burned, the rate of combustion is relatively high-speed, but the smoldering is continued for a longer time. Kudo (2010) evidenced that if the particle size is smaller, the flame spread rate dramatically increases. In addition, the more the size of the particle is small, the more the layers of oxidation and melting are thinner. This resulted that the combustion reaction being promptly expanded and transferring the heat of combustion is comparatively delayed if the particle size is small, and it takes a longer time to end up the combustion due to expanded smoldering. So far, most previous studies have addressed the combustible risks of magnesium as a metallic powder by looking at the phenomena of its explosive reaction; however, there have been few studies on fires of combustible metals, such as magnesium, even though the mechanism of combustible metal fires and explosions is different.”

Question #4: It mentioned that the experiment used the ‘magnesium powder’ rather than the real ‘magnesium dross’. In the meantime, a small-scale experiment was conducted. Can the results be applied to the real scale? How is the generality of the conclusions?

Response #4: Thank you for your questions. Yes, you are right. Apparently, our study would be difficult to apply to the real scale. The reason is that the real scale experiment would be a big challenge due to its risks of expanding fires and explosions. Secondly, despite the small-scale experiment, our experiment results showed almost the same patterns as the real fire patterns in magnesium recycling patterns. Moreover, our study shows magnesium dross would be dangerous in specific environmental conditions, such as with water and moisture. As your question, it would be difficult to be generalized due to those limitations. Thus, we added limitations of our study in the conclusion in lines 598-606 as follows:

“Despite the advantages of this study that suggested safety measures in magnesium recycling plants, there are a couple of limitations. The first is that the real scale experiment would be a big challenge due to the possibility of fire and explosion expansions. Secondly, despite the small-scale experiment, our experiment results showed almost the same patterns as the real fire patterns in magnesium recycling patterns. Also, different variations using dross, such as the amount of pure magnesium, % of moisture in the air, and different particle sizes and shapes, could determine the potential results of actual experiments. Thus, considering the other variations, detailed further studies and investigations would be required.”

Question #5: The language needs serious improvement. Many grammar problems make it very hard to understand.

Response #5: We went through the entire manuscript to eliminate grammatical mistakes.

Additional clarifications

In addition to the above comments, we have checked other parts of this manuscript using in red font.

We look forward to hearing from you in due time regarding our submission and to respond to any further questions and comments you may have.

Sincerely,

Kihun Nam

Reviewer 2 Report

This paper deals with magnesium combustion and intends to give advice for safety measures. The topic is very useful and interesting, but the article is strangely written. No scientific conclusions are drawn, only operational conclusions, but since I think it is the topic of Safety journal, I decide not to reject the paper. However, I have some remarks that should be answered before publication.

Introduction is well written with interesting references and gives a clear presentation of the context. However, I don’t understand why results on dust explosions (MIE, Kst, etc.) that are mentioned here are not used in the rest of the article. Results on differences on ignition energy and temperatures for different particle sizes for instance must be used to propose safety measures!

Section 2.1 is named “Theoretical Analysis …” but I don’t see any theory there. It is a literature review of magnesium characteristics and properties.

The cases studies presented are an historical presentation. They are very important and must be presented but not in that way, I don’t see the added value for the paper goal.

Performed experiments are based on a standard, which is suitable for the paper goal. Moreover, many characterization techniques are used. But the presentation of results is very poor, just a temperature plot and several pictures. I think these experiments are very important and must be presented and discussed in a better way.

The beginning of Discussion section, lines 422 to 464 is unnecessary there and is just a repetition of several things already stated before.

Title of Figure 17 is not suitable, authors do not propose a mechanism for magnesium fire. They propose measures and actions applicable to an already known fire mechanism. This is not for a me a fire mechanism since no reactions, no kinetics parameters are given for each stage.

To conclude, the topic is of interest and interesting experiments were performed. But the link with literature data (MIE, Kst, DSC and TGA analyses) has to be performed before publication.

Author Response

Reviewer #2

Question #1: Introduction is well written with interesting references and gives a clear presentation of the context. However, I don’t understand why results on dust explosions (MIE, Kst, etc.) that are mentioned here are not used in the rest of the article. Results on differences on ignition energy and temperatures for different particle sizes for instance must be used to propose safety measures!

Response #1: Thank you very much for agreeing with us and reminding us. We recognized the absence of them, and the contents regarding the results on dust explosions are added with several references in Section 2.1 in lines 124-146 as follows:

“Numerous studies have been carried out on assessing combustible characteristics by investigating the explosivity of magnesium dust and powder. Only a minority of studies consider fires of magnesium and dust and powder. The explosion of dust and power is sorted into the ignition and strength characteristics (Assael, 2010). Generally, the ignition characteristic is indicated by explosive range, MIT, MIE, and limiting oxygen concentration (LOC). The strength characteristic is denoted by P_max, maximum pressure rise rates (dP/dt_max), and flame propagation speed (Marc, 2010). NFPA 484 (2019) assesses explosion risk of combustible metal powder and dust with the factors of MIE, P_max, dP/dt_max, deflagration index (K_st), LOC, and MEC. Moreover, other studies related to magnesium dust explosion have addressed the explosive properties depending on different particle sizes through analysis on the ignition and strength characteristics (Nifuku et al., 2007; Li et al., 2008; Dehaan et al., 2012; Yuan et al., 2013; Mittal, 2014; Meng et al., 2020; Yashima, 2020).

Among the property factors, MIT (K) and MIE (mJ) are particularly considered as essential factors to assess explosive properties as well as fire risks. Previous studies have shown that the more the size of material particles is small, the more the MIE and MIT are decreased, which means the risk of explosion and fire is increased. For example, Nifuku et al. (2007) revealed that when magnesium particle size is 0-20 μm and 149-177 μm, the MIT and MIE is 786.15 K/4 mJ and 898.15 K/242 mJ, perspectively. Li et al. (2008) identified the MIE of magnesium particles with 6 μm is below 2mJ. Also, another experiment illustrated that MIE is more highly elevated rather than MIT, and 2 mJ of energy could be easily generated by electric sparks and fraction on the mechanical properties (Li et al., 2008). It means that the more magnesium particle is tiny, the more the risks of fire and explosion increase.”

Question #2: Section 2.1 is named “Theoretical Analysis …” but I don’t see any theory there. It is a literature review of magnesium characteristics and properties.

Response #2: Thank you very much for pointing this out. We agree with your opinion. We have reviewed more literature related to magnesium explosions and fires and added in lines 124-171. Moreover, we decided to change the title of Section 2.1 to “A review of literature on magnesium explosions and fires” that is more fit in the contents of Subsection 2.1 as follows:

“Numerous studies have been carried out on assessing combustible characteristics by investigating the explosivity of magnesium dust and powder. Only a minority of studies consider fires of magnesium and dust and powder. The explosion of dust and power is sorted into the ignition and strength characteristics (Assael, 2010). Generally, the ignition characteristic is indicated by explosive range, MIT, MIE, and limiting oxygen concentration (LOC). The strength characteristic is denoted by P_max, maximum pressure rise rates (dP/dt_max), and flame propagation speed (Marc, 2010). NFPA 484 (2019) assesses explosion risk of combustible metal powder and dust with the factors of MIE, P_max, dP/dt_max, deflagration index (K_st), LOC, and MEC. Moreover, other studies related to magnesium dust explosion have addressed the explosive properties depending on different particle sizes through analysis on the ignition and strength characteristics (Nifuku et al., 2007; Li et al., 2008; Dehaan et al., 2012; Yuan et al., 2013; Mittal, 2014; Meng et al., 2020; Yashima, 2020).

Among the property factors, MIT (K) and MIE (mJ) are particularly considered as essential factors to assess explosive properties as well as fire risks. Previous studies have shown that the more the size of material particles is small, the more the MIE and MIT are decreased, which means the risk of explosion and fire is increased. For example, Nifuku et al. (2007) revealed that when magnesium particle size is 0-20 μm and 149-177 μm, the MIT and MIE is 786.15 K/4 mJ and 898.15 K/242 mJ, perspectively. Li et al. (2008) identified the MIE of magnesium particles with 6 μm is below 2mJ. Also, another experiment illustrated that MIE is more highly elevated rather than MIT, and 2 mJ of energy could be easily generated by electric sparks and fraction on the mechanical properties (Li et al., 2008). It means that the more magnesium particle is tiny, the more the risks of fire and explosion increase.

In the other study, Yuan et al. (2013) conducted an experimental study using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) to identify combustion reactions with nitrogen and oxygen gases. In an experiment using a magnesium particle of 6 μm with a temperature rise rate of 325.15 K/min, the mass (%) was slightly increased between 603.15 and 773.15 K. This increase is interpreted as the starting oxidation and nitridation and it is rapidly reacted between 773.15 - 873.15 K. During the rapid reaction, the first peak was 863.15 ℃ in DSC analysis and the rapid oxidation was continued until the melting point reached 883.15 K. And then, the speed of the oxidation is slowed until it reaches 923.15 K. It means that the magnesium combustion proceeds in the order of slow-burning until it gets autoignition temperature, rapid oxidation and nitridation, a reach for the melting point of magnesium power, and a downturn of oxidation and nitridation.”

Question #3: The cases studies presented are an historical presentation. They are very important and must be presented but not in that way, I don’t see the added value for the paper goal.

Response #3: We checked Section 3. case studies. We agree with your comments. We wanted to show the cases and problems in handling magnesium and its compounds. First, we revised the title to “Recent Fires and explosions in a magnesium recycling plant in South Korea” that could help better understanding of the cases. And additional descriptions were updated in lines 308-311 as follows.

“Although the ignited materials were magnesium scrap collected and dross in two cases, the fires showed similar patterns of fire with the combustion process of pure magnesium entailing strong light, high heats and flames, and a long-term smouldering phase. Thus, a particular response measure is required to suppress magnesium fire.”

Moreover, we tried to explain magnesium recycling process that present two phases generating fire or explosion risks in lines 177-191 since the accidents are occurred from recycling plants.

“Magnesium fires have frequently occurred in the recycling processes of magnesium recycling plants in recent. Although there are differentiations of the recycling process depending on the countries and recycling techniques, the process, in general, consists of several phases, are i) scrap charging (Figure 4) & melting; ii) primary chemical refining; iii) finance transport; iv) secondary chemical refining; and v) ingot casting (Figure 3). Among them, the second and fourth phases, primary and secondary chemical refining, generate two types of by-products, well-known as dross. In particular, the primary chemical refining generates white dross that contains 15 – 70 % of pure magnesium and oxidized magnesium. The collected white dross is re-refined again to extract more magnesium, and it produces black dross containing 12 – 18 % of magnesium. This black dross is not available anymore after the second chemical refining and the dross is landfilled on the ground. Magnesium particles and/or a small amount of pure magnesium cannot be eliminated during the refining process. Nevertheless, there are no regulatory instruments to handle these white and black dross, as hazardous materials, and the magnesium recycling plants are exposed to the risks of fire and explosion.”

Question #4: Performed experiments are based on a standard, which is suitable for the paper goal. Moreover, many characterization techniques are used. But the presentation of results is very poor, just a temperature plot and several pictures. I think these experiments are very important and must be presented and discussed in a better way.

Response #4: Thank you for your comments. We provided updated descriptions that could support the absence of the result presentation and discussion in Results and Discussion.

Question #5: The beginning of Discussion section, lines 422 to 464 is unnecessary there and is just a repetition of several things already stated before.

Response #5: Thank you very much for your comments. We agree with your opinion and replaced the discussion of our study in lines 484-493 as follows:

“The causes of potential fires in magnesium recycling plants are classified into contact with ignitors and water (or moisture in the air). This study presents that the fire caused by ignitors induces a rapid expansion as shown in this study. On the other hand, the fire broke out by water or moisture has a longer time due to the smouldering following water reaction. The burning mechanism involving flames accompanies with oxidation and nitration on the surface of magnesium and the melting reaction inside. Through the XRD analysis on the ash, we identified different reactions inside and outside of burning magnesium imply that the more the magnesium particle is tiny and large amount, the more the melting status inside is extended and deeper. It means that the retarded combustion reaction influences the total duration of burning.”

Question #6: Title of Figure 17 is not suitable, authors do not propose a mechanism for magnesium fire. They propose measures and actions applicable to an already known fire mechanism. This is not for a me a fire mechanism since no reactions, no kinetics parameters are given for each stage.

Response #6: We revised the title of Figure 17, that is changed to Figure 18, as follows:

“XRD diffraction pattern of combustion’s by-products of magnesium powder”

Question #7: To conclude, the topic is of interest and interesting experiments were performed. But the link with literature data (MIE, Kst, DSC and TGA analyses) has to be performed before publication.

Response #7: Thank you very much for your suggestion. As you already mentioned literature of MIE, Kst, DSC and TGA analyses in the previous comment, we already provided an explanation in Subsection 2.1 in Comment 2.

Additional clarifications

In addition to the above comments, we have checked other parts of this manuscript using in red font.

We look forward to hearing from you in due time regarding our submission and to respond to any further questions and comments you may have.

Sincerely,

Kihun Nam

Author Response File: Author Response.pdf

Reviewer 3 Report

This is a good study. The conclusions are reasonable ( Fig. 17) , The scientific part needs improvement. The description of the Figures ( 11-16)  has to be expanded. It is not clear for example what Fig. 12 depicts. 
There are otehr areas not discussed. For example where carbonated magnesium is used.

Author Response

Reviewer #3

Question #1: This is a good study. The conclusions are reasonable (Fig. 17), The scientific part needs improvement. The description of the Figures (11-16) has to be expanded. It is not clear for example what Fig. 12 depicts. There are other areas not discussed. For example where carbonated magnesium is used.

Response #1: We thank you to acknowledge out work and review this manuscript. Following your comments, we found the deficiency in the descriptions for Figures 11-16. For Figure 11, we added explanation of the graph in lines 375 as follows:

“~2 min 20 sec until the temperature reached 1,473.6 K, ~.”

Another description is added in lines 377-379 since the result was similar with our other experiments. So we would like to show Figure is not the only result that the heat reached to 1,473.6 K in approximately 2 min 20 sec as follows:

“In our other experiment under the same conditions, elevating the temperature up to 1473.6 K at the beginning took about 2 min 11 sec (Lee and Nam, 2020).”

17. Lee, J. S. and Nam, K.H. (2020). Experimental study on the combustion characteristics of magnesium using infrared thermography and FE-SEM. 23(6), 927-934. https://doi.org/10.21289/KSIC.2020.23.6.927 (In Korean)

Additional descriptions of Figure 12 is added in lines 389-391 as follows:

“While the significant combustion reaction is burning with flames until 360 sec (Figure 12-c), the reaction is divided into burning with flames and burning with melting from 540 sec (Figure 12-d). Figure 12-e shows clearly separated reactions, burning with flame and burning with non-flame when the time is 720 sec.”

Figure 13 is newly attached with an explanation in lines 391-396 as follows.

“Figure 13 indicates that there is a difference in heat from the burning reaction with flames and melting. Thus, the entire combustion illustrates that the upper side of the burning point started with surface combustion with flames turned quickly into combustion with melting reaction; but the lower side of the burning point shows delayed combustion reaction with flames.”

We added detailed explanation of Figure 14 in lines 417-423 as follows:

“Figure 14 demonstrates the progress of combustion, that is proceed in order of number from ①, ②, ③. While magnesium power is burnt toward a direction to ①, where it is thinner than other areas, and then the combustion with flames are expanded to number ② area (the edge of magnesium dummy), the most major combustion reaction was melting directing number ③ area. The ash of burnt magnesium proves where was the combustion with flames by showing the white MgO and grey colour of carbide magnesium surrounding ① and ②.”

Through rechecked the contents, we found insufficient indication for Figure 15. Thus, we revised Figure 15 which is entire picture of Figure 16, and added an explanation in lines 444-445, 451-452, and 467-469 as follows:

“The by-product analysis using SEM and XRD presented differentiations in the by-products inside and outside. “

“Namely, there was no significant difference from the transformation of magnesium particles by high heat.”

“In addition, it means that the oxide reaction occurs on the surface of the material and the burning with melting reaction starts inside the material.”

Additional clarifications

In addition to the above comments, we have checked other parts of this manuscript using in red font.

We look forward to hearing from you in due time regarding our submission and to respond to any further questions and comments you may have.

Sincerely,

Kihun Nam

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The paper can be accepted.

Author Response

None.

Reviewer 2 Report

The quality of the paper has been improved in this revision.

However, I still have some comments before publication.

Section 4.2: the different measured quantities should be better stated.

Why is there a video recorder? Do authors measure flame velocity?

Line 383: what does mean 789.5 K after ignition?

Line 514: figure 19 instead of 17. I still have problems with the word mechanism used here, it is a qualitative description more than a mechanism. By the way, this comment was not clearly addressed in my previous review.

Similarly, the answer to my previous question 1 is not clear. Authors added references and improved this presentation, but I still do not see the link between the literature review and the authors results.

Author Response

Reviewer #2

Question #1: Section 4.2: the different measured quantities should be better stated. Why is there a video recorder? Do authors measure flame velocity?

Response #1: In this study, we used a video recorder to measure flame velocity. Moreover, a video recorder is used to record the overall experiment and combustion progress so that we can particularly analyze the entire progress of the combustion process efficiently and secure any significant phenomena that we can miss during our experiment. As you suggested, we improved the explanation to reduce any confusions in sub-section 4.2 in lines 366-370 as follows:

“In addition, we used a thermal infrared camera (model: Testo 890, 245.15 – 1475.15 K of thermal range, 7.5 – 14 μm of spectrum range) to identify heat distribution and evaporation on the surface of burning magnesium powder, and applied a general video recorder to observe the overall phenomena and progress of magnesium powder combustion.”

Question #2: Line 383: what does mean 789.5 K after ignition?

Response #2: Thank you pointing this out. We tried to say that the temperature of the dummy of magnesium power as soon as the magnesium power was ignited. We added more description to support insufficient description in lines 390-392 as follows:

“Figure 11 shows that the temperature of the magnesium powder-initiated burning is 789.5 K that is observed as soon as the dummy of magnesium powder ignited.”

Question #3: Line 514: figure 19 instead of 17. I still have problems with the word mechanism used here, it is a qualitative description more than a mechanism. By the way, this comment was not clearly addressed in my previous review.

Response #3: We checked the word ‘mechanism.’ Literally, the definition-well-known of mechanism is “a system of parts working together is a machine or a piece of machinery.” Another meaning of mechanism is established plan or process by which something takes place or is brought about.” (from Oxford languages) Thus, we took the second meaning of mechanism that we can show the process of magnesium fire and its safety measures in each stage. Considering that point, we identified it was not enough to use only fire mechanism, and we revised the title of figure 19 and some descriptions by adding supportive words. The title of figure 19 is changes to “Fire safety mechanism of magnesium dross generated from the recycling process.” Also, “mechanism” used in other places are also changed to fire safety magnesium in lines 16, 88-90, 535, and 592-595. These changes are coloured in a red font in the manuscript. We hope this answer would be appropriate to solve the problem.

Question #4: Similarly, the answer to my previous question 1 is not clear. Authors added references and improved this presentation, but I still do not see the link between the literature review and the authors results.

Response #4: Thank you for your comments. According to your comments, we reviewed and checked the connections between the literature review and the result. As we mentioned in the previous version, there have not been appropriate previous studies related to metal fires that are related to by-products generated from metal recycling plants. However, there has been significant differentiation between phenomena of metal powder explosion and fires. Considering the differentiation, we studied what must be different in fire safety and risk management in the metal recycling process through an experiment. Thus, we added more descriptions to help understanding the connections between the literature review and our results. The additional descriptions were updated in lines 173-181 as follows.

“Although the previous studies have tried to predict the risks of magnesium fires and explosions and manage potential ignitors, the studies are still focused on understanding the mechanism and risks of explosions. The mechanism of magnesium fire could be obviously different from the explosion’s one since magnesium combustion reactions with spreading fire and increasing heat varies according to melting, oxidation, and vaporization of the combustion process. It also creates various products formed during combustion of magnesium. Thus, fire risk management of magnesium requires establishing possible response measures considering scientifically assessed fire risks, potential ignitors, and understanding combustion phenomena of magnesium.”

Additional clarifications

In addition to the above comments, we have checked other parts of this manuscript using in red font.

We look forward to hearing from you in due time regarding our submission and to respond to any further questions and comments you may have.

Sincerely,

Kihun Nam, Author

Author Response File: Author Response.pdf

Reviewer 3 Report

corrections were acceptable 

Author Response

 None.